1. Field of the Invention
The present invention relates to new chelating resins and to a process for the treatment of acid mine drainage (AMD) to selectively recover valuable metals, in particular zinc and copper by novel functionalized resins, using the above chelating resins, to obtain discharge water free of toxic metals.
2. Description of Prior Art
Contamination of aqueous effluents by ions of toxic heavy metals poses a serious environmental problem for many industries. For example, Teachings from waste rock piles and mine tailings (called Acid Mine Drainage=AMD) contain various concentrations of zinc, cadmium and copper; selective removal of these metals would not only greatly reduce the environmental hazard of the discharge, but recovery of the valuable metals, along with possible application of the remaining AMD for the coagulation of municipal waters and wastewaters (by the action of Fe and Al ions remaining)[3], could help to finance the treatment process.
Acid mine drainage (AMD), generated wherever sulphide ores are exposed or processed, poses a serious environmental problem [1] as mentioned above. Essentially dilute sulphuric acid, it also contains a number of dissolved metal ions, the most common and abundant being (Fe(II), Fe(III), Al(III), as well as the more toxic heavy metal species (Cu(II), Zn(II), Ni(II) and Mn(II), along with As(III)), in varying concentrations depending on the source. The uncontrolled release of AMD threatens surrounding water resources (whether for ecosystems or human consumption), as well as wasting certain valuable metals which could otherwise be recovered.
Treatment of AMD by lime to neutralize it and precipitate the metals, followed by disposal of the resultant sludge, has been the standard practice [2]. This method does not enable the recovery of metal to be achieved; metal values are lost as metal hydroxide precipitate in the sludge. Moreover, this hydroxide sludge must be treated as hazardous waste. Alternately, metal recovery has the benefit of extending a natural resource and providing some revenue to offset the costs of treatment/disposal while decreasing the sludge volume.
If toxic heavy metals could be selectively removed, the solution of remaining Fe.sup.2+ /Fe.sup.3+, Al.sup.3+, Mg.sup.2+ etc. could then be used as a source of ferric chloride and alum, which could then be used in municipal wastewater treatment. Raw AMD has been used successfully as a coagulating agent in laboratory experiments [3].
Recovery of metals from AMD has been investigated by selective precipitation, through precise pH control, as (i) metal hydroxides and (ii) metal sulphides. Details of bench scale studies of the two options have been described [4]. A series of investigations aimed at the recovery of zinc from AMD from Mine Gallen have been completed in a joint McGill-Noranda project. Results to date have shown the possibility of recovering a significant proportion of zinc (up to 80%) as zinc sulphide of acceptable grade (55% Zn). However, the economics of this approach still needs to be improved.
Acid Mine Drainage (AMD)--the leaching of toxic and acidic salts (i.e. sulfates of cadmium, lead, mercury, nickel, copper and, particularly, zinc) from exposed mine tailings into the environment--is a major problem across Canada and worldwide. Various industrial processes also generate quantities of toxic heavy metals which, if not intercepted, are released into human and natural environments.
AMD is currently treated by liming (addition of calcium oxide) to precipitate most or all metal cations; however, the large volume of resulting sludge itself needs to be treated as hazardous waste. Alternatively, ion exchange/chelating resins could be used to actually recover the heavy metals and offset treatment and disposal costs. However, most such sorbents currently available are non-selective, thus removing even relatively innocuous and valueless iron, magnesium, and aluminum ions that are also present in large quantities; existing resins are also costly, chemically unstable under conditions of use (pH 2; ex. desulfonation of poly(styrenesulfonate), or cleavage of benzylic functional groups prepared from (chloromethyl)polystyrene), and can themselves be hazardous to manufacture (ex. reagents for chloromethylation of polystyrene are carcinogenic).
As well, acid mine drainage is very acidic (pH 2). The functional groups on most commercially-available polystyrene-based ion exchange resins are either directly bonded to the phenyl ring, or spaced by one carbon group (methylene spacer), making them susceptible to hydrolysis, damaging the resin.
Ion exchange resins are currently manufactured on a multi-ton scale for many applications, including water purification. Most are based on crosslinked polystyrene, Ps--H; (whose mechanical properties make it suitable for use in column beds, etc.) to which various chemical groups have been attached, especially sulphonate, (Ps--SO.sub.3.sup.- Na.sup.+) and quaternary ammonium (Ps-CH.sub.2 N.sup.+ Me.sub.3 Cl.sup.-) functionalities for relatively unselective cation- and anion-exchange resins respectively. A wide variety of other chemical groups can also be attached for various applications. Most such "functionalizations" are accomplished by the same route used to make the most common anion-exchange resins: chloromethylation of crosslinked polymer matrix (Ps--H.fwdarw.Ps--CH.sub.2 --Cl), followed by substitution of chloride with a nucleophile "X" to give a functional polymer of general structure "Ps-CH.sub.2 -X".
Ion-exchange resins are funtionalized polymers that are currently manufactured on a multi-ton scale for many applications, including water purification (ex. deionization). Most such resins are composed of crosslinked polystyrene ("Ps--H", good mechanical properties make it suitable for use in columns, beds, etc.) to which various chemical groups have been attached, especially sulfonate ("Ps--SO.sub.3.sup.-+ Na") and quaternary ammonium ("Ps--CH.sub.2 --NM.sub.e3.sup.+- Cl") functionalities for relatively unselective cation- and anion-exchange resins respectively. Many other chemical groups have also been attached to date, for a wide variety of applications; most such "functionalizations" are accomplished by the same route used to make the more common anion-exchange resins: i.e. chloromethylation of the crosslinked polymer matrix ("Ps--H" to "Ps--CH.sub.2 --Cl"), followed by substitution of chloride with a nucleophile "X" to give a functional polymer of general structure "Ps--CH.sub.2 --X"..sup.3 However, this current route has some problems: first, the chloromethylation step uses carcinogenic reagents; second, the "Ps--CH.sub.2 --X" products are often unstable under conditions of their use (ex. in the presence of heat, base and/or nucleophiles), because the connection between functional group and polymer backbone is through a fragile "benzylic" chemical bond.
An ion exchange resin consists of a chemically-inert polymer matrix, such as polystyrene ("Ps"), holding a functional group chemically bound to the polymer backbone. The functional group may be anionic, such as sulphonate, with an exchangeable cationic counter-ion such as sodium (Ps--SO.sub.3.sup.-+ Na), or cationic, such as quaternary ammonium salt, with an anionic mobile counter-ion such as chloride (Ps--CH.sub.2 NMe.sub.3.sup.+- Cl). It is the counter-ion which can be exchanged for ions of like charge in solution. Thus, a cation-exchange resin exchanges one cation for another; for example, binding a heavy metal cation while releasing sodium into solution.
A third type of resin, of interest to this project, is a chelating resin. It is sometimes also loosely referred to as a type of ion exchange resin, although it does not necessarily release one ion as it binds another. Chelation (Greek chelae="claw") takes place when the lone-pair electrons of several electron-rich heteroatoms (O, N, S: "Lewis bases") in the same molecule are able to "coordinate" (form a loose bond) simultaneously to an electron-poor entity (metal cation: a "Lewis acid"), forming a stable structure for an overall strong association (see below). Chelating groups can be very selective for specific ions, according to the identities, numbers and positions of the coordinating heteroatoms. Attaching such a chelation compound to an insoluble powder of crosslinked polymer would allow removal of both chelator and complexed ion from a liquid by simple filtration.
Some Common Chelating Compounds ##STR2##
Cation exchange resins have been used extensively as separation tools in process industries. Metal ion species are adsorbed by the resin in exchange for generally Na.sup.+ or H.sup.+. The process has also been extensively used in the treatment of both municipal and industrial wastewaters [5, 6]. Simple cation exchange resins have also been used for the treatment of acid mine drainage [7]. Being quite unselective, their application however has been limited to the bulk removal of all dissolved metals in order to purify the water (use of cation and anion exchange resins in sequence gives "deionized", similar to "distilled" water). In recent years, new varieties of resins have been synthesized, such as "Chelex-100" (structure 27, FIG. 2c, similar to "iminodiacetate", FIG. 1), which are able to form chelates with adsorbed metal ions [8, 9]. Though relatively inert towards such non-targeted ions as Ca.sup.+2, Mg.sup.+2 or Na.sup.+ [8], such current commercial chelating resins are less able to discriminate between transition (heavy) metals, such as zinc and iron ions. Nevertheless, these have been used for the recovery of precious metals from the effluents of certain industries [9].
The metals absorbed by such resins can generally be recovered using eluent liquids rich in other cations, such as sodium (e.g. brine) or protons (e.g. nitric and sulphuric acids). The resulting low volume of metal concentrate may be further used or processed in solution form, or the metal ion eventually precipitated as hydroxide, and/or electrodeposited as the element.
Ion exchange/chelating resins are easy to handle, non-toxic, safely transportable, and can be regenerated repeatedly for multiple re-use. The ideal resin should be easy and inexpensive to manufacture; have a high affinity, capacity and selectivity for the target; be easy to recover, regenerate and recycle; be mechanically and chemically stable to the conditions of its use and regeneration. Crosslinked polystyrene is a proven inert and stable matrix for ion exchange resins and other functional polymers for many applications, and is particularly appropriate for columns of all sizes. The design for a zinc-versus-iron-selective sorbent thus depends on the choice of an appropriate chelating functional group.
Most commercial resins are based on crosslinked polystyrene, Ps-H; (whose mechanical properties make it suitable for use in column beds, etc.) to which various chemical groups have been attached, especially sulphonate (Ps-SO.sub.3.sup.- Na.sup.+) and quaternary ammonium (Ps--CH.sub.2 N.sup.+ Me.sub.3 Cl.sup.-) functionalities for cation and anion exchange resins respectively. Such functional groups are relatively unselective. Selective functional groups can be introduced to prepare ion exchange resins for the selective uptake of specific metals. The principle determining selectivity which is used in the present work is hard/soft formalism [10]. In this, metal ions are classified as hard or soft "Lewis acids" (electron donors) and the ligand (functional group) as hard or soft "Lewis bases" (electron acceptors). A ligand is a hard (Lewis) base if it is non-polarizable and is a soft base if it is polarizable. Sulphur and phosphorus-containing ligands are polarizable and therefore soft. Oxygen is harder, and the oxygen-containing carboxylate and hydroxyl groups are consequently hard ligands. A metal ion is a soft (Lewis) acid if it has easily polarizable electrons, or has a low charge, while a hard metal ion has high charge or valence electrons which are not polarizable. The "HSAB" (Hard-Soft-Acid-Base) rule states that soft ligands tend to form complexes with soft metal ions, and hard ligands form complexes with hard ions.
According to this classification Fe(III) ion is hard and not complexed with a soft ligand. Zn(II) ion is relatively soft and can be complexed with a soft ligand. On this basis ion exchange resins containing soft ligands are potentially selective to remove zinc leaving ferric ions in solution. The chelating groups of interest will have sulphur and possibly nitrogen in them (nitrogen is slightly soft). The sulphur-containing groups will be in the form of thiols, possibly also having thiolates, thioketones, thioethers, thiazoles, etc., in the chelant. The nitrogen groups will be in the form of amines, which are protonated at low pH's, resulting in positively-charged, hydrophilic groups which can make the resin more hydrophilic (wettable). Other nitrogen-containing functionalities such as ringed structures (e.g. pyridine) may be included. Since oxygen-containing chelants have a tendency to complex iron, these should be avoided.
Ligands which prefer to bind zinc over iron are also likelier to bind other soft cations, such as those of cadmium, lead, mercury, silver, copper, gold, etc.--all of which are toxic and/or valuable.
As well as being selective for the metal(s) of choice, it is important to have chelating functional groups which are non-hydrolyzable (stable to acid), and which are strongly bonded to the polymer backbone, so that they are not removed during usage.
Much work has been done in the past on finding metal-selective chelating groups. Generally the areas of development are analytical chemistry and biomedical applications (metal poisoning treatments) [11]. The work done in these fields will be useful in the development of metal-selective chelating resins.